Basically, what the W1 limiter is doing (and indeed, all other limiters, since REAPER has a few on-board as well) is turning down all of the louder parts of the audio to be in-line with the quieter parts, and then adding gain to bring the whole track up in volume. Think of it as a compressor, since it kind of is one. The ceiling control is how “loud” the maximum volume is allowed to be on a given track before the limiter just tells it “ok, you can go no louder”.
Generally you’d want to keep this somewhere around -0.5dB like DemonUnicorns suggests. If you’re setting the threshold to more than -7, probably try using the volume controls instead first, since that may add noticeable (audible) gain to the track at that point. (Though in mixing your tracks, you might actually want this effect, but for the purposes of customs, probably not. Also W1 is pretty great with keeping tracks relatively clean, unlike the JS Limiter within REAPER.)
